This disclosure relates to the monitoring of motion, breathing, heart rate and sleep state of humans in a convenient and low-cost fashion, and more particularly to an apparatus, system, and method for acquiring, processing and displaying the corresponding information in a easily understandable format.
Monitoring of sleep patterns, heart rate and respiration during sleep is of interest for many reasons from clinical monitoring of obstructive and central sleep apnea in both adults and young children, to ensuring healthy sleep patterns in young babies. For example, infants which are born prematurely often have immature cardiorespiratory control which can cause them to stop breathing for 15-20 seconds, or to breathe shallowly. This is referred to as apnea of prematurity, and often persists for two to three months after birth. Periodic breathing (in which the amplitude of respiration rises and falls over several minutes) is also common in babies born prematurely. In such infants, it is also useful to monitor heart rate as a low heart rate (bradycardia) can be used as a warning signal that the baby is not receiving sufficient oxygen.
In adults, common sleep disordered breathing syndromes include obstructive sleep apnea and central sleep apnea. In obstructive sleep apnea, the upper airway collapses, restricting the flow of air to the lungs, even in the presence of ongoing respiratory effort. Obstructive sleep apnea can also cause characteristic changes in heart rate, which may be detrimental to the subject. Obstructive sleep apnea has a high prevalence in the adult population, affecting about 2-4% of adults over the age of 40. Obstructive events lead to a reduced flow of air to the lungs, and subsequently a lowering of oxygen level in the blood. Central sleep apnea is less common than obstructive sleep apnea in adults, and is distinguished by a complete loss of respiratory effort, which leads to a loss of air to the lungs, and eventually a lowering of oxygen in the blood. In both central and obstructive sleep apnea, the body's natural defense mechanisms will be stimulated by the oxygen desaturation, and eventually increase respiratory effort sufficient to restore airflow. However, this is often accompanied by an arousal (which can be observed in the person's electroencephalogram) which either wakes the person up momentarily, or brings them into a lighter stage of sleep. In either event, the person's sleep is disrupted, and they experience poor quality sleep, which often leads to excessive daytime sleepiness.
Other common sleep disorders in adults, whose effects are not related to respiration are Periodic Limb Movements Disorder (PLMD) and Restless Legs Syndrome (RLS). In PLMD, a subject makes characteristic repetitive movements (usually of the leg) every 30-40 seconds, leading to sleep disruption due to frequent awakenings. In RLS, the subject has an overwhelming desire to move or flex their legs as they fall asleep, again leading to disrupted sleep patterns. Monitoring of these unusual body movements is important to confirming the diagnosis of these conditions and initiating treatment.
The most common adult sleep disorder is insomnia which is defined as a difficulty in initiating or maintaining sleep. Chronic insomnia is estimated to affect about 10% of the American population. However, at present full clinical evaluation of sleep patterns relies on electroencephalograph (EEG) monitoring, often requiring a hospital stay. There is a need for simpler methods of assessing sleep patterns for adults in the home environment. For example, evidence has shown that sleep deprivation adversely alters the balance of leptin and ghrelin, two hormones which are significantly involved with the body's appetite control system. Voluntary sleep deprivation over a period of time (due to lifestyle choice) has been correlated with increased Body-Mass-Index (an indicator of obesity). Hence, objective measurement and control of sleep patterns may play a role in weight management.
Moreover, sleep is of particular important to young children. Infants spend more time asleep than awake in their first three years, emphasizing its crucial importance in development. Sleep is important for physical recuperation, growth, maturing of the immune system, brain development, learning, and memory. Conversely, infants who do not receive sufficient sleep or who sleep poorly often display poor mood, as well as having an adverse effect on their parents' sleep patterns. Indeed it is estimated that 20-30% of children under the age of 3 years have common sleep problems such as frequent night-wakings, and difficulty falling asleep on their own. Studies have shown that parents can help their babies achieve good sleep patterns through a variety of behavioral approaches. A non-invasive safe sleep monitor can assist in adopting such behavioral approaches. Automated collection of sleep information can help parents in assuring their children are sleeping adequately. For example, a system which monitors night-time sleep and daytime naps can provide information in the form of a visual sleep log which can be stored and visualized over a period of time (e.g., using a world wide web interface on a personalized page). The sleep monitor can also track sleep fragmentation (e.g., frequent awakenings during night-time sleep), which is correlated with infant contentment. Finally, characteristic changes in breathing, heart rate, and movement may be associated with night-time urination and defecation in infants, and hence can be used to alert parents to change diapers.
In adults, measurements of heart rate and breathing rate during sleep can be used as clinical markers for continuous health monitoring. For example, elevated breathing rates can be linked to forms of respiratory distress or diseases such as chronic obstructive pulmonary disease which require increased respiratory effort. It has been shown in clinical studies that a particular type of breathing pattern, referred to as Cheyne-Stokes respiration or periodic breathing, is a marker for poor prognosis in people with heart disease. Simultaneous measurement of respiration and cardiac activity can also allow evaluation of a phenomenon called respiratory sinus arrhythmia (RSA) in which the heart rate speeds up and slows down in response to each breath. The amplitude of this coupling effect is typically stronger in young healthy people, and therefore can be used as another health marker. Heart rate changes during sleep can also provide useful clinical information—elevated heart rates can be an indicator of systemic activation of the sympathetic nervous system, which can be associated with sleep apnea or other conditions. Furthermore, a common clinical problem is to monitor response to treatments aimed at stabilizing heart rhythm. For example, a common cardiac arrhythmia is atrial fibrillation (AF), in which the upper chambers of the heart beat irregularly. Consequently the heart rate is irregular and elevated. Common treatments for AF include pharmacological and surgical approaches, and a goal of the doctor is to provide follow-up monitoring to look for a reoccurrence of the arrhythmia. Non-invasive low-cost monitoring of heart rate during sleep is a useful mechanism to provide doctors with a means of providing such monitoring follow-up for this condition, and other cardiac arrhythmias.
Accordingly, a method, system or apparatus which can reliably monitor sleep patterns, breathing and heart rate during sleep, and motion during sleep would have utility in a variety of settings.
A variety of techniques have been disclosed in the background art for addressing the need for respiratory, cardiac and sleep monitoring. Respiratory monitoring is currently carried out primarily in a hospital environment using a variety of approaches. A common method for measuring respiratory effort uses inductance plethysmography, in which a person wears a tightly fitting elastic band around their thorax, whose inductance changes as the person breathes in and out. This technique has become the most widely used respiration monitoring technique in sleep medicine. A severe limitation of the method from a convenience point of view is that the person has to wear a band, and remains connected to the associated electronic recording device via wires.
An alternative system for measuring respiratory effort is to use impedance pneumography, in which the impedance change of the thorax is measured. This technique is often used in clinical infant apnea monitors, which generate an alarm in a baby monitor when no breathing is detected. In order to detect the breathing signal, electrodes must be attached to a sleeping infant. More generally, there are a number of commercial products available which use impedance measurements across the baby's chest to detect central apnea (e.g., the AmiPlus Infant Apnea Monitor produced and marketed by CAS Medical Systems). The limitation of this technology is that it requires electrodes to be attached to the baby, has an active electrical component, and needs to be used with caution as the wires can cause strangulation if not properly fitted.
Heart rate during sleep can be measured using conventional surface electrocardiogram measurements (typically referred to as a Holter monitor), in which a person typically wears three or more electrodes. A limitation of this method is the need to wear electrodes and the associated electronic recording device. Heart rate fitness monitors record heart rate by also measuring surface electrocardiogram, typically using a wearable chest band which has integrated electrodes. Again, there is the need to wear the device and also the accompanying signal collector (typically a wrist watch style device). Heart rate during sleep can also be measured using pulse oximetry, in which a photoplethysmogram is collected at the finger or ear. There is a characteristic variation in the pulse photoplethysmogram signal which corresponds to each beat of the heart.
Integrated systems for collecting heart rate and respiration using combinations of the techniques discussed above for heart rate and respiratory effort have been developed. In one commercial product, contact ECG and inductance plethysmograph sensors have been embedded in a custom-designed jacket. The cost of providing such a wearable system is relatively high, and the system requires contact sensors.
One indicator of sleep status is the degree of motion while lying down. Motion during sleep can be detected by wrist-worn accelerometers, such as those commercially marketed by MiniMitter as “Actiwatch®”. These use microelectronic accelerometers to record limb movement during sleep. A limitation of this technology is the requirement for the individual to wear a device, and the fact that it is not integrated with simultaneous breathing and cardiac monitoring, which limits the physiological usefulness of such measurements. Motion can also be detected using under-mattress piezoelectric sensors, which produce a voltage spike when pressure is applied to the mat, and hence can detect movement.
Various approaches to measuring heart rate, respiration, and motion in a non-contact fashion have been described. One approach is to use optical interferometry to provide a non-contact method for determining respiration, cardiac activity and motion. However, a limitation of their invention is that the optical signals are blocked by clothes or bedding materials. The processing required to obtain and differentiate breathing, cardiac and motion elements is unclear. A second approach is to use ultrasonic waves to detect motion. A limitation of this approach is that signal-to-noise ratio can be poor due to low reflection, and respiration, motion and cardiac signals can not be collected simultaneously. A further non-contact measurement technique for assessing bodily motion is to use continuous wave radar (using electromagnetic radiation in the radio frequency range) in detecting respiration and heartbeat.
Limitations of previous methods to obtain physiological data using these non-contact methods include various sensor limitations (e.g., obstruction by bed clothes, poor signal-to-noise ratios, or the need for too large an antenna). Furthermore, the background art does not provide methods for extracting useful “higher-level” physiological status, such as breathing rate, cardiac rhythm status, sleep state, respiratory distress, or evidence of sleep disturbed breathing. The current disclosure also possesses advantages related to the fact that it requires very low levels of transmitted radio-frequency power (e.g., less than 0 dBm), can be made in a small size (e.g., the sensor can be 5 cm×5 cm×5 cm or less in size), can be battery powered, and is safe for human use.